bims-misrem Biomed News
on Mitochondria and sarcoplasmic reticulum in muscle mass
Issue of 2020‒05‒10
fifteen papers selected by
Rafael Antonio Casuso Pérez
University of Granada


  1. Aging (Albany NY). 2020 May 05. 12
    Lamb DA, Moore JH, Mesquita PHC, Smith MA, Vann CG, Osburn SC, Fox CD, Lopez HL, Ziegenfuss TN, Huggins KW, Goodlett MD, Fruge AD, Kavazis AN, Young KC, Roberts MD.
      We examined if resistance training affected muscle NAD+ and NADH concentrations as well as nicotinamide phosphoribosyltransferase (NAMPT) protein levels and sirtuin (SIRT) activity markers in middle-aged, untrained (MA) individuals. MA participants (59±4 years old; n=16) completed 10 weeks of full-body resistance training (2 d/wk). Body composition, knee extensor strength, and vastus lateralis muscle biopsies were obtained prior to training (Pre) and 72 hours following the last training bout (Post). Data from trained college-aged men (22±3 years old, training age: 6±2 years old; n=15) were also obtained for comparative purposes. Muscle NAD+ (+127%, p<0.001), NADH (+99%, p=0.002), global SIRT activity (+13%, p=0.036), and NAMPT protein (+15%, p=0.014) increased from Pre to Post in MA participants. Additionally, Pre muscle NAD+ and NADH in MA participants were lower than college-aged participants (p<0.05), whereas Post values were similar between cohorts (p>0.10). Interestingly, muscle citrate synthase activity levels (i.e., mitochondrial density) increased in MA participants from Pre to Post (+183%, p<0.001), and this increase was significantly associated with increases in muscle NAD+ (r2=0.592, p=0.001). In summary, muscle NAD+, NADH, and global SIRT activity are positively affected by resistance training in middle-aged, untrained individuals. Whether these adaptations facilitated mitochondrial biogenesis remains to be determined.
    Keywords:  NAD +; NADH; aging; muscle; resistance training
    DOI:  https://doi.org/10.18632/aging.103218
  2. Am J Physiol Endocrinol Metab. 2020 May 05.
    Castro-Sepulveda M, Jannas-Vela S, Fernández-Verdejo R, Avalos D, Tapia G, Villagrán C, Quezada N, Zbinden-Foncea H.
      BACKGROUND: Disturbances in skeletal muscle lipid oxidation might induce ectopic fat deposition and lipotoxicity. Nevertheless, the cellular mechanisms that regulate skeletal muscle lipid oxidation have not been fully determined. We aimed to determine whether there was an association between relative whole-body lipid oxidation with mitochondrial size and with mitochondria-sarcoplasmic reticulum interactions in the skeletal muscle.METHODS: Twelve healthy males were included (mean [standard deviation], 24.7 [1.5] years, 24.4 [2.6] kg/m2). The respiratory quotient (RQ) was used to estimate relative lipid oxidation at rest, and during exercise (50% VO2max, 600 kcal expended). A skeletal muscle biopsy was obtained from the Vastus Lateralis at rest. Transmission electron microscopy was used to determine mitochondrial size and mitochondria-sarcoplasmic reticulum interactions (≤50 nm of distance between organelles). Protein levels of fusion/fission regulators were measured in skeletal muscle by western blot.
    RESULTS: Resting RQ and exercise RQ associated inversely with intermyofibrillar mitochondrial size (r = -0.66 and r = -0.60, respectively, P < 0.05). Resting RQ also associated inversely with the percentage of intermyofibrillar mitochondria-sarcoplasmic reticulum interactions (r = -0.62, P = 0.03). Finally, intermyofibrillar mitochondrial size associated inversely with lipid droplet density (r = -0.66, P = 0.01) but directly with mitochondria fusion/fissionratio(Mfn 2/Fis 1) (r = 0.61, P = 0.03).
    CONCLUSION: Our results show that whole body lipid oxidation is associated to skeletal muscle intermyofibrillar mitochondrial size - fusion phenotype - and mitochondria-sarcoplasmic reticulum interactions in non-diabetic human.
    Keywords:  Respiratory quotient; fat oxidation; lipid droplet; mitochondria dynamic; organelles dynamic
    DOI:  https://doi.org/10.1152/ajpendo.00025.2020
  3. J Sport Health Sci. 2020 May 04. pii: S2095-2546(20)30039-9. [Epub ahead of print]
    Powers SK, Deminice R, Ozdemir M, Yoshihara T, Bomkamp MP, Hyatt H.
      The first report demonstrating that prolonged endurance exercise promotes oxidative stress in humans was published more than four decades ago. Since this discovery, many ensuing investigations have corroborated the fact that muscular exercise increases the production of reactive oxygen species (ROS) and results in oxidative stress in numerous tissues including blood and skeletal muscles. Although several tissues may contribute to exercise-induced ROS production, it is predicted that muscular contractions stimulate ROS production in active muscle fibers and that skeletal muscle is a primary source of ROS production during exercise. This contraction-induced ROS generation is associated with 1) oxidant damage in several tissues (e.g., increased protein oxidation and lipid peroxidation), 2) accelerated muscle fatigue, and 3) activation of biochemical signaling pathways that contribute to exercise-induced adaptation in the contracting muscle fibers. While our understanding of exercise and oxidative stress has advanced rapidly during the last decades, questions remain about whether exercise-induced increases in ROS production are beneficial or harmful to health. This review will address this issue by discussing the site(s) of oxidant production during exercise and detailing the health consequences of exercise-induced ROS production.
    Keywords:  Hormesis; Oxidants; Radicals; Reactive oxygen species; Skeletal muscle
    DOI:  https://doi.org/10.1016/j.jshs.2020.04.001
  4. FASEB J. 2020 May 05.
    Buch BT, Halling JF, Ringholm S, Gudiksen A, Kjøbsted R, Olsen MA, Wojtaszewski JFP, Pilegaard H.
      The aim of the study was to investigate the impact of autophagy inhibition on skeletal muscle mitochondrial function and glucose homeostasis in young and aged mice. The transcriptional co-activator PGC-1α regulates muscle oxidative phenotype which has been shown to be linked with basal autophagic capacity. Therefore, young and aged inducible muscle-specific PGC-1α knockout (iMKO) mice and littermate lox/lox controls were used in three separate experiments performed after either saline or colchicine injections on two consecutive days: (1) Euthanization in the basal state obtaining skeletal muscle for mitochondrial respirometry, (2) whole body glucose tolerance test, and (3) in vivo insulin-stimulated 2-deoxyglucose (2-DG) uptake into skeletal muscle. Muscle PGC-1α was not required for maintaining basal autophagy flux, regardless of age. Colchicine-induced inhibition of autophagy was associated with impairments of skeletal muscle mitochondrial function, including reduced ADP sensitivity and altered mitochondrial redox balance in both young and aged mice. Colchicine treatment reduced the glucose tolerance in aged, but not young mice, and similarly in iMKO and lox/lox mice. Colchicine reduced insulin-stimulated 2-DG uptake in soleus muscle in aged mice, independently of PGC-1α, and without affecting insulin-regulated phosphorylation of proximal or distal mediators of insulin signaling. In conclusion, the results indicate that autophagy regulates the mitochondrial ADP sensitivity and redox balance as well as whole body glucose tolerance and skeletal muscle insulin sensitivity in aged mice, with no additional effects of inducible PGC-1α deletion.
    Keywords:  ADP sensitivity; ROS; aging; autophagy
    DOI:  https://doi.org/10.1096/fj.201903113RR
  5. Am J Physiol Cell Physiol. 2020 May 06.
    Luo D, Edwards S, Smeuninx B, McKendry J, Nishimura Y, Perkins M, Philp A, Joanisse S, Breen L.
      The role of dysregulated intracellular creatine metabolism in disuse atrophy is unknown. In this study, skeletal muscle biopsy samples were obtained after 7-days of unilateral leg immobilization (IMMOB) and the non-immobilized control limb (CTRL) of 15 healthy males (23.1 ± 3.5 yrs). Samples were analyzed for fibre-type cross-sectional area (CSA) and creatine transporter (CreaT) at the cell membrane periphery (MEM) or intracellular (INT) areas, via immunoflouresence microscopy. Creatine kinase (CK) and AMP-activated protein kinase (AMPK) were determined via immunoblot. PCr, Cr and ATP were measured via enzymatic analysis. Body composition and maximal isometric knee extensor strength were assessed before and after disuse. Leg strength and fat-free mass were reduced in IMMOB (~32% and 4%, respectively; P<0.01 for both). Type II fibre CSA was smaller (~12%; P=0.028) and intramuscular PCr lower (~13%; P=0.015) in IMMOB vs. CTRL. CreaT protein was greater in Type I fibres in both limbs (P<0.01). CreaT was greater in IMMOB vs. CTRL (P < 0.01) and inversely associated with PCr concentration in both limbs (P < 0.05). MEM CreaT was greater than the INT CreaT in Type I and II fibres of both limbs (~14% for both; P<0.01 for both). Type I fibre CreaT tended to be greater in IMMOB vs. CTRL (P=0.074). CK was greater, and phospho-to-total AMPKThr172 tended to be greater, in IMMOB vs. CTRL (P=0.013 and 0.051, respectively). These findings suggest that modulation of intracellular creatine metabolism is an adaptive response to immobilisation in young healthy skeletal muscle.
    Keywords:  Atrophy; Creatine Metabolism; Imobilization; Skeletal Muscle
    DOI:  https://doi.org/10.1152/ajpcell.00072.2020
  6. J Appl Physiol (1985). 2020 May 07.
    Toth MJ, Voigt TB, Tourville TW, Prior SM, Guigni BA, Schlosberg AV, Smith IB, Forest TJ, Kaufman PA, Wood ME, Rehman H, Dittus K.
      Exercise has numerous benefits for patients with cancer, but implementation is challenging because of practical and logistical hurdles. This study examined whether neuromuscular electrical stimulation (NMES) can serve as a surrogate for classical exercise by eliciting an exercise training response in skeletal muscle of women diagnosed with breast cancer undergoing chemotherapy. Patients (n=22) with histologically-confirmed, stage I, II or III breast cancer scheduled to receive neoadjuvant or adjuvant chemotherapy were randomized to 8 weeks of bilateral neuromuscular electrical stimulation (NMES; 5 days/week) to their quadriceps muscles or control. Biopsy of the vastus lateralis was performed at baseline and after 8 weeks of intervention to assess muscle fiber size, contractility and mitochondrial content. Seventeen patients (8 control/9 NMES) completed the trial and were included in analyses. NMES promoted muscle fiber hypertrophy (P<0.001), particularly in fast-twitch, myosin heavy chain (MHC) IIA fibers (P<0.05), and tended to induce fiber type shifts in MHC II fibers. The effects of NMES on single muscle fiber contractility were modest and it was unable to prevent declines in function in MHC IIA fibers. NMES did not alter intermyofibrillar mitochondrial content/structure, but was associated with reductions in subsarcolemmal mitochondria. Our results demonstrate that NMES induces muscle fiber hypertrophy and fiber type shifts in MHC II fibers, but had minimal effects on fiber contractility and promoted reductions in subsarcolemmal mitochondria. Further studies are warranted to evaluate the utility of NMES as an exercise surrogate in cancer patients and other conditions.
    Keywords:  exercise; hypertrophy; muscle contraction
    DOI:  https://doi.org/10.1152/japplphysiol.00203.2020
  7. Philos Trans R Soc Lond B Biol Sci. 2020 Jun 22. 375(1801): 20190414
    Molenaars M, Daniels EG, Meurs A, Janssens GE, Houtkooper RH.
      Lifespan in eukaryotic species can be prolonged by shifting from cellular states favouring growth to those favouring maintenance and stress resistance. For instance, perturbations in mitochondrial oxidative phosphorylation (OXPHOS) can shift cells into this latter state and extend lifespan. Because mitochondria rely on proteins synthesized from nuclear as well as mitochondrial DNA, they need to constantly send and receive messages from other compartments of the cell in order to function properly and maintain homeostasis, and lifespan extension is often dependent on this cross-compartmental signalling. Here, we describe the mechanisms of bi-directional mitochondrial cross-compartmental signalling resulting in proteostasis and longevity. These proteostasis mechanisms are highly context-dependent, governed by the origin and extent of stress. Furthermore, we discuss the translatability of these mechanisms and explore therapeutic developments, such as the antibiotic studies targeting mitochondria or mitochondria-derived peptides as therapies for age-related diseases such as neurodegeneration and cancer. This article is part of the theme issue 'Retrograde signalling from endosymbiotic organelles'.
    Keywords:  longevity; mitochondria; proteostasis; retrograde signalling
    DOI:  https://doi.org/10.1098/rstb.2019.0414
  8. J Physiol. 2020 May 05.
    Knudsen JR, Li Z, Persson KW, Li J, Henriquez-Olguin C, Jensen TE.
      HIGHLIGHTS: AMPK-dependent Raptor Ser792 phosphorylation does not influence mTORC1-S6K1 activation by intense muscle contraction. α2 -AMPK activity-deficient mice have lower contraction-stimulated protein synthesis Increasing glycogen activates mTORC1-S6K1 independently of AMPK α2 Normalizing muscle glycogen content rescues reduced protein synthesis in AMPK-deficient mice ABSTRACT: Objective The mammalian Target of Rapamycin Complex 1 (mTORC1)-S6K1 signalling pathway regulates muscle growth-related protein synthesis and is antagonized by AMP-activated protein kinase (AMPK) in multiple cell types. Resistance exercise stimulates skeletal muscle mTORC1-S6K1 and AMPK signalling and post-contraction protein synthesis. Glycogen inhibits AMPK and has been proposed as a pro-anabolic stimulus. The aim of this study was to investigate how muscle mTORC1-S6K1 signalling and protein synthesis respond to resistance exercise-mimicking contraction in the absence of AMPK and with glycogen-manipulation. Methods Resistance exercise-mimicking unilateral in situ contraction of m. quadriceps femoris on anaesthetized wild-type and dominant negative α2 AMPK kinase dead transgenic (KD-AMPK) mice, measuring muscle mTORC1 and AMPK signalling immediately (0h) and 4h post-contraction, and protein-synthesis at 4h. Muscle glycogen manipulation by 5-day oral gavage of the glycogen phosphorylase inhibitor CP316819 and sucrose (80 g L-1 ) in the drinking water prior to in situ contraction. Results The mTORC1-S6K1 and AMPK signalling axes were co-activated immediately post-contraction, despite potent AMPK-dependent Raptor Ser792 phosphorylation on the mTORC1 subunit Raptor. KD-AMPK muscles displayed normal mTORC1-S6K1 activation at 0h and 4h post-exercise, but impaired contraction-stimulated protein synthesis 4h post-contraction. Pharmacological/dietary elevation of muscle glycogen content augmented contraction-stimulated mTORC1-S6K1-S6 signalling and rescued the reduced protein synthesis-response in KD-AMPK to WT levels. Conclusions mTORC-S6K1 signalling is not influenced by α2 -AMPK during or after intense muscle contraction. Elevated glycogen augments mTORC1-S6K1 signalling. α2 -AMPK-deficient KD-AMPK mice display impaired contraction-induced muscle protein synthesis, which can be rescued by normalizing muscle glycogen content. This article is protected by copyright. All rights reserved.
    DOI:  https://doi.org/10.1113/JP279780
  9. Skelet Muscle. 2020 May 06. 10(1): 14
    Yang S, Loro E, Wada S, Kim B, Tseng WJ, Li K, Khurana TS, Arany Z.
      PGC-1 (peroxisome-proliferator-activated receptor-γ coactivator-1) alpha is a potent transcriptional coactivator that coordinates the activation of numerous metabolic processes. Exercise strongly induces PGC-1alpha expression in muscle, and overexpression of PGC-1alpha in skeletal muscle activates mitochondrial oxidative metabolism and neovascularization, leading to markedly increased endurance. In light of these findings, PGC-1alpha has been proposed to protect from age-associated sarcopenia, bone loss, and whole-body metabolic dysfunction, although these findings have been controversial. We therefore comprehensively evaluated muscle and whole-body function and metabolism in 24-month-old transgenic mice that over-express PGC-1alpha in skeletal muscle. We find that the powerful effects of PGC-1alpha on promoting muscle oxidative capacity and protection from muscle fatigability persist in aged animals, although at the expense of muscle strength. However, skeletal muscle PGC-1alpha does not prevent bone loss and in fact accentuates it, nor does it have long-term benefit on whole-body metabolic composition or insulin sensitivity. Protection from sarcopenia is seen in male animals with overexpression of PGC-1alpha in skeletal muscle but not in female animals. In summary, muscle-specific expression of PGC-1alpha into old age has beneficial effects on muscle fatigability and may protect from sarcopenia in males, but does not improve whole-body metabolism and appears to worsen age-related trabecular bone loss.
    DOI:  https://doi.org/10.1186/s13395-020-00231-8
  10. J Cell Mol Med. 2020 May 04.
    Heyne E, Schrepper A, Doenst T, Schenkl C, Kreuzer K, Schwarzer M.
      In heart failure, high-fat diet (HFD) may exert beneficial effects on cardiac mitochondria and contractility. Skeletal muscle mitochondrial dysfunction in heart failure is associated with myopathy. However, it is not clear if HFD affects skeletal muscle mitochondria in heart failure as well. To induce heart failure, we used pressure overload (PO) in rats fed normal chow or HFD. Interfibrillar mitochondria (IFM) and subsarcolemmal mitochondria (SSM) from gastrocnemius were isolated and functionally characterized. With PO heart failure, maximal respiratory capacity was impaired in IFM but increased in SSM of gastrocnemius. Unexpectedly, HFD affected mitochondria comparably to PO. In combination, PO and HFD showed additive effects on mitochondrial subpopulations which were reflected by isolated complex activities. While PO impaired diastolic as well as systolic cardiac function and increased glucose tolerance, HFD did not affect cardiac function but decreased glucose tolerance. We conclude that HFD and PO heart failure have comparable effects leading to more severe impairment of IFM. Glucose tolerance seems not causally related to skeletal muscle mitochondrial dysfunction. The additive effects of HFD and PO may suggest accelerated skeletal muscle mitochondrial dysfunction when heart failure is accompanied with a diet containing high fat.
    Keywords:  heart failure; high-fat diet; interfibrillar/subsarcolemmal mitochondria; pressure overload; skeletal muscle
    DOI:  https://doi.org/10.1111/jcmm.15325
  11. Cell Metab. 2020 May 05. pii: S1550-4131(20)30196-0. [Epub ahead of print]31(5): 886-887
    Schuler MH, Hughes AL.
      In this issue of Cell Metabolism, Herkenne et al. (2020) show that the mitochondrial fusion protein OPA1 promotes angiogenesis independent of its function in mitochondrial dynamics, identifying a key new therapeutic target to prevent vascular growth during development and tumor formation.
    DOI:  https://doi.org/10.1016/j.cmet.2020.04.014
  12. J Sci Med Sport. 2020 Apr 04. pii: S1440-2440(19)31694-9. [Epub ahead of print]
    Kilen A, Bay J, Bejder J, Breenfeldt Andersen A, Bonne TC, Larsen PD, Carlsen A, Egelund J, Nybo L, Mackey AL, Olsen NV, Aachmann-Andersen NJ, Andersen JL, Nordsborg NB.
      OBJECTIVES: Military-, rescue- and law-enforcement personnel require a high physical capacity including muscular strength. The present study hypothesized that 9 weeks of volume matched concurrent short frequent training sessions increases strength more efficiently than less frequent longer training sessions.DESIGN: A randomized training intervention study with functional and physiological tests before and after the intervention.
    METHODS: Military conscripts (n=290) were assigned to micro-training (four 15-min strength and four 15-min endurance bouts weekly); classical-training (one 60-min strength and one 60-min endurance training session weekly) or a control-group (two 60-min standard military physical training sessions weekly).
    RESULTS: There were no group difference between micro-training and classical-training in measures of strength. Standing long jump remained similar while shotput performance was reduced (P≤0.001) in all three groups. Pull-up performance increased (P≤0.001) in micro-training (7.4±4.6 vs. 8.5±4.0 repetitions, n=59) and classical-training (5.7±4.1 vs. 7.1±4.2 repetitions, n=50). Knee extensor MVC increased (P≤0.01) in all groups (micro-training, n=30, 11.5±8.9%; classical-training, n=24, 8.3±11.5% and control, n=19, 7.5±11.8%) while elbow flexor and hand grip MVC remained similar. Micro-training increased (P≤0.05) type IIa percentage from 32.5±11.0% to 37.6±12.3% (n=20) and control-group increased (P≤0.01) type IIax from 4.4±3.0% to 11.6±7.9% (n=8). In control-group type I, fiber size increased (P≤0.05) from 5121±959μm to 6481±2084μm (n=5). Satellite cell content remained similar in all groups.
    CONCLUSIONS: Weekly distribution of low-volume concurrent training completed as either eight 15-min bouts or two 60-min sessions of which 50% was strength training did not impact strength gains in a real-world setting.
    Keywords:  Military; Muscle biopsies; Muscle fiber composition and size; Satellite cells
    DOI:  https://doi.org/10.1016/j.jsams.2020.03.013
  13. Front Cell Dev Biol. 2020 ;8 239
    Bakula D, Scheibye-Knudsen M.
      Maintaining mitochondrial health is emerging as a keystone in aging and associated diseases. The selective degradation of mitochondria by mitophagy is of particular importance in keeping a pristine mitochondrial pool. Indeed, inherited monogenic diseases with defects in mitophagy display complex multisystem pathologies but particularly progressive neurodegeneration. Fortunately, therapies are being developed that target mitophagy allowing new hope for treatments for previously incurable diseases. Herein, we describe mitophagy and associated diseases, coin the term mitophaging and describe new small molecule interventions that target different steps in the mitophagic pathway. Consequently, several age-associated diseases may be treated by targeting mitophagy.
    Keywords:  aging; autophagy; interventions; mitophaging; mitophagy; monogenic disorders
    DOI:  https://doi.org/10.3389/fcell.2020.00239
  14. Redox Biol. 2020 Apr 18. pii: S2213-2317(20)30159-2. [Epub ahead of print] 101531
    Gomez-Cabrera MC, Arc-Chagnaud C, Salvador-Pascual A, Brioche T, Chopard A, Olaso-Gonzalez G, Viña J.
      Muscle mass and strength are very important for exercise performance. Training-induced musculoskeletal injuries usually require periods of complete immobilization to prevent any muscle contraction of the affected muscle groups. Disuse muscle wasting will likely affect every sport practitioner in his or her lifetime. Even short periods of disuse results in significant declines in muscle size, fiber cross sectional area, and strength. To understand the molecular signaling pathways involved in disuse muscle atrophy is of the utmost importance to develop more effective countermeasures in sport science research. We have divided our review in four different sections. In the first one we discuss the molecular mechanisms involved in muscle atrophy including the main protein synthesis and protein breakdown signaling pathways. In the second section of the review we deal with the main cellular, animal, and human atrophy models. The sources of reactive oxygen species in disuse muscle atrophy and the mechanism through which they regulate protein synthesis and proteolysis are reviewed in the third section of this review. The last section is devoted to the potential interventions to prevent muscle disuse atrophy with especial consideration to studies on which the levels of endogenous antioxidants enzymes or dietary antioxidants have been tested.
    DOI:  https://doi.org/10.1016/j.redox.2020.101531
  15. Acta Physiol (Oxf). 2020 May 04.
    Skattebo Ø, Calbet JAL, Rud B, Capelli C, Hallén J.
      We analysed the importance of systemic and peripheral arteriovenous O2 difference (a- v - O2 and a-vf O2 difference, respectively) and O2 extraction fraction for maximal oxygen uptake ( V ˙ O2max ). Fick law of diffusion and the Piiper and Scheid model were applied to investigate whether diffusion vs perfusion limitations vary with V ˙ O2max . Articles (n=17) publishing individual data (n=154) on V ˙ O2max , maximal cardiac output ( Q ˙ max ; indicator-dilution or Fick method), a- v - O2 difference (catheters or Fick equation) and systemic O2 extraction fraction were identified. For the peripheral responses, group-mean data (articles: n=27; subjects: n=234) on leg blood flow (LBF; thermodilution), a-vf O2 difference and O2 extraction fraction (arterial and femoral venous catheters) were obtained. Q ˙ max and two-LBF increased linearly by 4.9-6.0 L·min-1 per 1 L·min-1 increase in V ˙ O2max (R2 =0.73 and R2 =0.67, respectively; both P<0.001). The a- v - O2 difference increased from 118-168 mL·L-1 from a V ˙ O2max of 2-4.5 L·min-1 followed by a reduction (second-order polynomial: R2 =0.27). After accounting for a hypoxemia-induced decrease in arterial O2 content with increasing V ˙ O2max (R2 =0.17; P<0.001), systemic O2 extraction fraction increased up to ~90% ( V ˙ O2max : 4.5 L·min-1 ) with no further change (exponential decay model: R2 =0.42). Likewise, leg O2 extraction fraction increased with V ˙ O2max to approach a maximal value of ~90-95% (R2 =0.83). Muscle O2 diffusing capacity and the equilibration index Y increased linearly with V ˙ O2max (R2 =0.77 and R2 =0.31, respectively; both P<0.01), reflecting decreasing O2 diffusional limitations and accentuating O2 delivery limitations. In conclusion, although O2 delivery is the main limiting factor to V ˙ O2max , enhanced O2 extraction fraction (≥90%) contributes to the remarkably high V ˙ O2max in endurance-trained individuals.
    Keywords:  Arteriovenous oxygen difference; Cardiac output; Exercise; Leg blood flow; Limiting factors; Maximal oxygen uptake; Oxygen diffusion; Stroke volume
    DOI:  https://doi.org/10.1111/apha.13486